This invention relates to an electronic copier or printer of the type which uses an addressable image writing array (write bar) to form images on a moving photoreceptor surface. More particularly, the invention is directed towards a control system for compensating for errors in imaged scan lines due to mechanical vibration present in the photoreceptor.
In document reproduction systems which incorporate scanning illumination systems (moving illumination sources, reflectors, projection lens) to project an image onto a moving photoreceptor, it is known that undesirable exposure strobing can result from mechanical vibrations of the various components comprising the optical system.
U.S. Pat. No. 4,699,497 describes a method and apparatus for compensating for the effects of vibration of a scanning illumination lamp as it accelerates through a scan cycle. The compensation for this system relies on increasing the natural frequency of the illumination lamp to nullify the effects of the lamp vibration.
Another common cause of strobing in prior art, light lens systems is the non-uniform (vibrational) motion of the photoreceptor, whether of the drum or belt type, in copending application Ser. No. 910,708 assigned to the same assignee as the present invention, there is disclosed an optical transmission filter which is placed in the optical path. The filter has specified transmittance characteristics which result in reduction or elimination of the stobing.
Mechanical vibration of the photoreceptor is also a problem in imaging systems of the type in which an image writing array is addressed by an image signal input. The array, or image bar, generates line-like image ray patterns, represenative of the image signal input, on the surface of a photoreceptor, Examples of image bar type of printers are found in U.S. Pat. No. 4,477,175, where the image bar is composed of a plurality of individually activated illumination elements (LEDs), and in U.S. Pat. No. 4,389,659 where the image bar is composed of a plurality of individually activated electro-optic electrodes. For these types of printing systems, photoreceptor vibration can cause periodic modulation in the gray regions, a phenomenon known as banding. Periodic exposure modulation in raster-generated images is the digital analog of exposure strobing caused by mechanical vibrations in scanning-type copiers. Resultant output copies are subject to solid area exposure non-uniformity banding in half tones and variations in line width.
The present invention is therefore directed towards a mechanism for detecting photoreceptor vibrations in an image bar type printing system and for generating signals to control the on/off time and the radiance of the image bar to compensate for the periodic modulation. More particularly, the invention relates to an image bar printing apparatus including an image write bar, a photoconductive member adapted for movement though an exposure station, image signal source means connected to said image write bar, said write bar generating a light output representaive of said signal source input, a lens array cooperating with said image bar to form line-like images on said photoconductive member as the member moves through said exposure station, a pixel clock for controlling the on-off time of said image write bar, means for generating electrical signals representative of at least velocity motion changes in the photoconductive member due to vibrational forces, and logic and control circuitry for operating on said electrical signals and for generating at least a first output signal which adjusts the time-dependent radiance of said image bar to compensate for said velocity motion changes
The following disclosures appear to be relevant.
U.S. Pat. No. 4,414,583 to Hooker, discloses a scanned light beam imaging apparatus including a frequency modulator (FM) 80 and an amplitude modulator (AM) 61 for controlling the movement of a light beam, orthogonally with respect to a scan direction for breaking up any unscanned or overlapped interstices between adjacent scan lines.
U.S. Pat. No. 4,307,409 to Greenig et al discloses a beam feedback synchronization system, wherein character clock signals are synchronized with electrical signals outputted from a photodetector, in response to light entering fiber optic elements, to compensate for variations in the speed of a light beam across a fiber optic array.